What sort of cognitive hypothesis is a derivational theory of grammar?

This paper has two closely related aims. The main aim is to lay out one specific way in which the derivational aspects of a grammatical theory can contribute to the cognitive claims made by that theory, to demonstrate that it is not only a theory’s posited representations that testable cognitive hypotheses derive from. This requires, however, an understanding of grammatical derivations that initially appears somewhat unnatural in the context of modern generative syntax. The second aim is to argue that this impression is misleading: certain accidents of the way our theories developed over the decades have led to a situation that makes it artificially difficult to apply the understanding of derivations that I adopt to modern generative grammar. Comparisons with other derivational formalisms and with earlier generative grammars serve to clarify the question of how derivational systems can, in general, constitute hypotheses about mental phenomena